EMI reduction device and assembly

ABSTRACT

An EMI reduction device coupled between to a printed circuit board (PCB) assembly and a heat sink is herein disclosed. The PCB assembly includes a processor core that is the source of unintentional electromagnetic interference (EMI). The EMI reduction device is attached to a heat sink which is positioned over the processor core such that it capacitively couples emissions from the processor core to a grounding plane resident in the PCB assembly, thereby reducing the unintentional EMI. Simultaneously, the EMI reduction device is able to maintain thermal contact with the heat sink.

FIELD OF THE INVENTION

[0001] The present invention relates generally to electronic systems.More particularly, the invention relates to an electromagneticinterference (EMI) reduction device.

BACKGROUND OF THE INVENTION

[0002] A trend in the electronics industry is to generate smaller andfaster electronic devices. As a result, these devices consume more powerand hence, produce more heat. Excessive heat can cause significantdamage to an electronic device thereby reducing its life. For thisreason, various techniques are employed to eliminate or dissipate theheat generated from the electronic device. A heat sink is one suchtechnique. A heat sink is thermally coupled to the electronic device andas such, allows the heat to flow from the electronic device through theheat sink to the surrounding open space.

[0003] In addition, the operation of these electronic devices isaccompanied by the generation of electromagnetic radiation or energy.The emissions of the electromagnetic radiation can cause significantelectronic interference or noise with other surrounding electroniccircuitry. To reduce such disturbances, the source of these emissions isusually suppressed. However, the EMI suppression problem is furthercomplicated when the electronic device is coupled to a heat sink. Theheat sink acts as an antenna for the EMI energy thereby amplifying theEMI energy.

[0004] A common solution for suppressing EMI energy is to ground theheat sink. FIG. 1 illustrates an exemplary grounded heat sink. There isshown a heat sink 100 coupled to a grounded electronic device 102 thatis mounted onto a printed circuit board (PCB) 104. In this example, thegrounded electronic device 102 is a processor core having a siliconlogic die 106. A thermal compound 108, such as a dielectric material, isplaced between the heat sink 100 and the grounded electronic device 102so that thermal contact is maintained and the heat generated by thedevice 102 is transferred to the heat sink 100.

[0005] Mounting fences 110 are positioned on four opposite sides of thePCB 104 in order to ground the heat sink 100. The mounting fences 110are attached to the heat sink 100 and the PCB 104, which acts in thiscase as a ground plane. The mounting fences 110 provide a Faraday shieldaround the device 102 in order to shield the EMI energy generated fromthe clock circuitry internal to the processor 102 from damaging adjacentcomponents and from releasing EMI energy outside of the heat sink 100.

[0006] Additional EMI suppression is provided by four sets of groundingpads 112 a-112 d on the PCB 104 that surround the processor 102, asshown in FIG. 2. The grounding pads 112 minimize or ground EMI noisegenerated by the switching of all the component pins under maximumcapacitive load. Thus, the combination of the grounding pads and thegrounded heat sink dissipates the heat from the processor core andgrounds the EMI energy generated by the processor core.

[0007] As the internal clock speed of processors increase, theseprocessors will require more extensive EMI reduction or grounding. Insome cases, the grounded heat sink approach may not be suitable toeffectively eliminate the EMI emissions since this approach has a longergrounding path. As such, grounding may need to be performed closer tothe silicon logic die in order to create a shorter grounding path thateffectively shields the EMI energy emanating from the processor core.

[0008] Accordingly, there is a need for an EMI reduction technique thatcan minimize or eliminate EMI energy generated from a processor corecloser to the source of the emissions and in such a manner that is costefficient and easy to manufacture.

SUMMARY OF THE INVENTION

[0009] The present invention pertains to an EMI reduction device that iscoupled between a printed circuit board (PCB) assembly and a heat sink.The PCB assembly includes a processor core that is the source of a largeamount of electromagnetic interference. In a first embodiment, the EMIreduction device is fastened to a heat sink which is positioned over theprocessor core such that it capacitively couples the emissions from theprocessor while maintaining thermal contact with the heat sink.

[0010] In a first embodiment, the EMI reduction device has aelectrically-conductive rectangular frame that includes a die aperturepositioned in the center of the device, four mounting clip tabs, andfour spring contact fingers. The die aperture allows the processor logicdie to be in direct physical contact with the bottom surface of the heatsink thereby maintaining thermal contact between the processor logic dieand the heat sink. The four mounting clip tabs are positioned on theouter periphery of the reduction device and are used to clip the EMIreduction device to the heat sink.

[0011] The spring contact fingers are located on each of the four sidesof the processor. The spring contact fingers are the only portion of theEMI reduction device that maintains physical contact with the PCBassembly. This is to ensure that the EMI reduction device does notprotrude directly onto the PCB assembly yet remains in close proximityto the processor. As such, the EMI reduction device is not in directelectrical contact with the PCB assembly. The EMI reduction devicegrounds the processor emissions by capacitively coupling them to theground plane present in the PCB assembly.

[0012] A pair of heat sink retention clips is used to secure the heatsink to the PCB assembly. The heat sink retention clips are insertedthrough mounting holes on the heat sink in a downward direction throughthe mounting holes of the EMI clip and on the PCB assembly.

[0013] The placement of the EMI clip in close proximity with theprocessor is beneficial since it creates a shorter grounding paththereby effectively reducing the EMI energy emanating from the processordirectly at the source. In addition, the EMI clip is able to accomplishthis while maintaining thermal contact with the heat sink. Furthermore,performing the EMI reduction at the processor is more economical thanincorporating an EMI reduction technique into a chassis assembly thathouses the PCB assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For a better understanding of the nature and objects of theinvention, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

[0015]FIG. 1 is a schematic view of a prior art grounded heat sinkassembly for a printed circuit board;

[0016]FIG. 2 is a top plan view of the printed circuit board used in thegrounded heat sink assembly shown in FIG. 1;

[0017]FIG. 3 is a perspective view of a processor module in accordancewith a first embodiment of the present invention;

[0018]FIG. 4 is an exploded view of the components of the processormodule shown in FIG. 3;

[0019]FIG. 5 illustrates the processor module positioned on a printedcircuit board as part of computer system in accordance with theembodiments of the present invention;

[0020]FIG. 6 is a front perspective view of the printed circuit boardassembly shown in FIGS. 3 and 4;

[0021]FIG. 7 is a top plan view of the printed circuit board shown inFIG. 6;

[0022]FIG. 8 is a perspective view of the bottom side of the EMI clipshown in FIGS. 3 and 4;

[0023]FIG. 9 is a top plan view of the EMI clip positioned onto theprinted circuit board assembly in accordance with a first embodiment ofthe present invention;

[0024]FIG. 10 is a perspective view of the heat sink shown in FIGS. 3and 4 in accordance with the embodiments of the present invention;

[0025]FIG. 11 is cross-sectional view of the processor module takenalong plane AA shown in FIG. 3;

[0026]FIG. 12 is a rear perspective view of the heat sink with the EMIclip attached thereto in accordance with the first embodiment of thepresent invention;

[0027]FIG. 13 is an isometric view of a printed circuit board havingspring fingers mounted onto the PCB assembly in accordance with a secondembodiment of the present invention; and

[0028]FIG. 14 is an isometric view of a printed circuit board havingspring washers in accordance with a third embodiment of the presentinvention.

[0029] Like reference numerals refer to corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0030]FIGS. 3 and 4 depict a processor module 120 having a PCB assembly122, an EMI clip or reduction device 124, a heat sink 126, and a pair ofheat sink retention clips 128 a-128 b. The PCB assembly 122 includes aprocessor core 130. The EMI clip 124 is positioned onto the PCB assembly122 surrounding the processor core 130 and is used to efficiently shieldthe EMI emissions generated from the processor core 130. The EMI clip124 is removably mounted to the heat sink 126. The heat sink 126 isthermally coupled to the PCB assembly 122 and is used to transfer theheat from the electronic components to the heat sink 126. A pair of heatsink retention clips 128 is used to fasten the heat sink 126 to the PCBassembly 122.

[0031] The processor module 120 can be removably mounted onto amotherboard or other type of circuit board. For example, as shown inFIG. 5, the processor module 120 is positioned onto a motherboard 127through a processor retention assembly 129. The motherboard 127 can bepart of a computer system, subsystem, or the like.

[0032] Preferably, the PCB assembly 122 is a Single Edge ContactCartridge 2 (SECC2) manufactured by the Intel Corporation. The SECC2supports “slot” type processors. A slot is a connector that is residenton a motherboard and which supports the Intel P6 microprocessor bus.Previously, microprocessors were mounted onto a motherboard through asocket. In a slot-type processor, the processor is placed on a PCB knownas a single edge contact (SEC). The SEC has pins on an edge of the cardwhich is inserted into a 242-pin slot on the motherboard . This pinconstruction is otherwise known as an edge finger connection.

[0033] A more detailed discussion of the SECC2 can be found in IntelCorporation, Single Edge Contact Connector 2 (S.E.C.C.2) ThermalInterface Material Functional Requirements, Order No: 244458-001,November 1998; Intel Corporation, Single Edge Contact Cartridge 2(S.E.C.C.2) Heat Sink Attachment and Heat Sink Functional RequirementsOrder Number: 244456-001, Nov. 23, 1998; and Intel Corporation,S.E.C.C.2 Heat Sink Installation and Removal Process, Order Number244454-001, December 1998 which are hereby incorporated by reference asbackground information.

[0034]FIG. 6 illustrates the SECC2 122. There is shown a printed circuitboard 132 and a cover 134. The PCB 132 is preferably a substrateconstructed of a multilayer plastic laminate, such as a plastic landgrid array or organic land grid array processor core substrates. Thesubstrate 132 has an edge finger connection. In addition, the substrate132 includes a ground plane. The cover 134 is used to protect the PCB132 and is removably mounted to the cover 134 through a set of snaps136. However, it should be noted that the technology of the presentinvention is not constrained to the SECC2, to any particular circuitboard, or to any particular circuit board layout.

[0035]FIG. 7 illustrates the PCB 132 in further detail. There is aprocessor core 130 that includes a silicon logic die 138. The PCB 132includes four attachment holes 140 a-140 d through which the heat sinkretention clips 128 are fit so that the heat sink can be attached to thePCB assembly 122. In addition, surrounding the logic die 138, there areother electronic components, such as but not limited to cache memory,resistors, capacitors, and the like. However, in this embodiment of thePCB 132, there are no grounding pads.

[0036]FIG. 8 illustrates the EMI clip 124. The EMI clip 124 hasrectangular-shaped frame including a die aperture 150, four mountingclip tabs 152 a-152 d, four attachment holes 154 a-154 d, and fourspring contact fingers 156 a-156 d. The die aperture 150 is of arectangular shape and is located in the center of the EMI clip 124. TheEMI clip 124 is positioned over the logic die 138 so that the dieaperture 150 allows the logic die 138 to be in direct physical contactwith the surface of the heat sink 126.

[0037] The four mounting clip tabs 152 a-152 d are used to attach theEMI clip 124 to the heat sink 126. The mounting clip tabs 152 a-152 dare located on the outer periphery of the EMI clip 124 and are situatedin each corner of the top side of the EMI clip 124.

[0038] The mounting holes 154 a-154 d are located on the surface of thetop side of the EMI clip 124 and each mounting hole 154 is located at aparticular corner of the EMI clip 124. Each of the four mounting holes154 a-154 d are aligned with a respective mounting hole 140 on the PCB132 and with a respective mounting hole on the heat sink. The retentionclips 128 are inserted through the mounting holes 166 on the heat sink126 in a downward direction through the mounting holes 154 on the EMIclip 124 through the mounting holes 140 on the PCB 132.

[0039] The four spring contact fingers 156 a-156 d are situated on thebottom side of the EMI clip 124 and allow the EMI clip 124 to maintainphysical contact on four sides of the processor core 130. The contactfingers 156 are the only portion of the EMI clip 124 that is in physicalcontact with the PCB 132. The spring fingers 156 ensure that the frameof the EMI clip 124 does not protrude directly onto any of thecomponents of the PCB 132. Direct contact with the processor substrate132 would damage the electronic components positioned under the EMI clip124. As such, each of the spring contact fingers 156 is positioned onthe PCB 132 in an area where there are no electronic components.

[0040] The shape, size and dimension of the EMI clip 124 is tailored forthe particular processor substrate 132 to which the clip 124 is coupled.FIG. 9 illustrates the placement of the EMI clip 124 onto a SECC2 122.For this embodiment, the EMI clip 124 is approximately 48.97 mm by 54.35mm. The die aperture 150 is 19 mm by 18 mm and is constructed such thatthere is a 6 mm distance between the outer edge of the processoraperture to the outer edge of the processor logic die. It should benoted that the present invention is not constrained to an EMI clip 124having the particular shape, size and dimensions described herein andone skilled in the art can easily modify the design of the EMI clip 124to suit other PCBs or the like.

[0041] The electromagnetic bandwidth of the processor core can extend toinclude multiple times the advertised processor clock speed. In the caseof an Intel Pentium II processor with an internal clock speed of 500MHz, the EMI bandwidth can extend to 2500 MHz or more. The EMI clip 124reduces portions of that bandwidth with varying shielding effectivenessand can vary depending on the mechanical dimensions of the EMI clip 124.Another modifier to EMI reduction is the mechanical pressure that exitsboth on the heat sink 126 and on the processor core 130. The higher thepressure the more effective the capacitive coupling becomes and hencethe EMI reduction.

[0042] Preferably, the EMI clip 124 is constructed of thin steel sheetmetal. However, the present invention is not constrained to anyparticular type of metal and can be constructed of any material, such asbut not limited to stainless steel, beryllium copper, phospher bronze,hardened steel, spring steel, and the like.

[0043] Referring to FIG. 12, the bottom side of the heat sink 126, thatside which touches the PCB 132, is coated with a dielectric layer thatensures that the EMI clip 124 does not short the electronic componentsmounted on the surface of the PCB 132. An example of such a dielectriccoating is powder paint, in particular, the epoxy power coatings basedon epoxy resins.

[0044] The placement of the EMI clip 124 over the processor core 130 andin the manner described above is effective in reducing the EMI emissionsfrom the processor core 130. Although, the EMI clip 124 is not in directelectrical contact with the processor core 130, its close proximity tothe processor core 130 capacitively couples these emissions to theground plane of the substrate 132 and through a shorter grounding path.As a result, the emissions are minimized or eliminated directly at theprocessor core 130.

[0045]FIGS. 10 and 11 illustrates the heat sink 126. The heat sink 126has a number of fins 160, a base portion 162, and two skirts 164 a-164b. The fins 160 project outwardly and upwardly from the base portion162. The base portion 162 extends over and covers the processor core 130thereby providing the maximum amount of thermal contact area between theprocessor core 130 and the heat sink 126. The two skirts 164 a-164 bextend laterally and downwardly from the base portion 162 and extendsover and covers opposite sides of the PCB assembly 122.

[0046] The fins 160 are eliminated where the heat sink retention clips128 are positioned (see FIGS. 3 and 4) into two groves situated on thetop surface of the base portion of the heat sink 126. The groves includemounting holes 166 that enable the heat sink retention clips 128 to fitthrough the top surface of the base portion 162. The retention clips 128are inserted through the mounting holes 166 on the heat sink 126 in adownward direction through the mounting holes 154 on the EMI clip 124through the mounting holes 140 on the PCB 132.

[0047] The heat sink retention clips 128 are made from a flexiblematerial that is non-electrically conductive, such as plastic. The heatsink retention clips 128 are secured at one end in any appropriatemanner, and in the present example by barbed ties. Preferably, the heatsink retention clips 128 are those heat sink retention clips designedfor the SECC2 and which are manufactured by ITW Fastex.

[0048]FIG. 12 illustrates the bottom side of the base portion 162 of theheat sink 126. There is shown the EMI clip 124 mounted to the baseportion 162 of the heat sink 126. Each mounting clip 152 is fitted overonto a corresponding groove 170 on the base portion of the heat sink126.

[0049] The heat sink 126 is made of aluminum that is chromate conversioncoated and is electrically conductive. A thermal compound 168, such as adielectric layer, is placed on the bottom side of the base portion 162of the heat sink 126. This dielectric layer is used to provide highdielectric capacity and to provide a high thermal conductivity betweenthe processor logic die 138 and the heat sink 126. An example of such acompound is MCM-STRATE® manufactured by Power Devices, Inc. However, thepresent invention is not constrained to any particular type of materialand other materials having the same properties can also be used.

[0050] The foregoing description, for purposes of explanation, usedspecific nomenclature to provide a thorough understanding of theinvention. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice theinvention. In other instances, well known circuits and devices are shownin order to avoid unnecessary distraction from the underlying invention.Thus, the foregoing descriptions of specific embodiments of the presentinvention are presented for purposes of illustration and description.They are not intended to be exhaustive or to limit the invention to theprecise forms disclosed, obviously many modifications and variations arepossible in view of the above teachings. The embodiments were chosen anddescribed in order to best explain the principles of the invention andits practical applications, to thereby enable others skilled in the artto best utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the followingclaims and their equivalents.

[0051] In particular, one skilled in the art can alter the EMI clipdescribed above to include additional spring fingers in order to providemore capacitive contacts between the EMI clip and the substrate. Inanother alternate embodiment, the EMI clip can be used in conjunctionwith grounding pads that are positioned on the substrate. In yet anotherembodiment, the EMI clip can be made an integral part of the heat sinkand not a separate device. Furthermore, the EMI clip can be made withoutthe dielectric coating on the bottom surface thereby making electricalcontact with the PCB. Alternatively, the EMI clip can be applied to aprocessor that is mounted directly onto a motherboard and is notconstrained to edge finger connection devices.

[0052]FIG. 13 illustrates a second embodiment of the present invention.In this embodiment, there is no EMI clip 124 rather spring fingers 180a-180 d mounted directly onto the printed circuit board 132′.Preferably, a spring finger 180 is mounted on each side surrounding theprocessor core 130′. As shown in FIG. 13, there are four spring fingers180 a-180 d mounted directly onto the circuit board 132′. However, thenumber of spring fingers 180 and their position of the circuit board 132is not a limitation of this embodiment.

[0053] A heat sink is mounted directly over the circuit board 132′ andcoupled with retention clips in the manner described above. The bottomsurface of the heat sink will be in direct physical contact with theprocessor die 138′ and the top surface of the spring fingers 180. Assuch, the emissions from the processor die 138′ are capacitively coupledto the ground plane in the circuit board 132′ and suppressed directly atthe source of the emissions.

[0054]FIG. 14 illustrates a third embodiment of the present invention.In this embodiment, the ground plane of the circuit board 132″ isextended to and through the mounting holes 140 a″-140 d″ on the circuitboard 132″. A spring washer 182 a-182 d is mounted in each mounting hole140 a″-140 d″. A heat sink is mounted over the top surface of thecircuit board 132″.

[0055] The spring washers 182 are formed of an electrically conductivematerial, such as but not limited to thin sheet steel metal and areshaped as a compression coil. The spring washers 182 serve to ground theemissions from the processor core 130″ while ensuring that the placementof the heat sink on top of the circuit board 132″ does not damage theprocessor core 130″ and the electronic components mounted thereon.

[0056] Each spring washer 182 has a hollow core that is wide enough toallow a retention clip to fit through it thereby allowing the heat sinkto be fastened to the circuit board 132″ in the manner described above.When the heat sink is fastened to the circuit board 132″, the springwashers 182 are compressed by the weight of the heat sink and the bottomsurface of the heat sink is in direct physical contact with theprocessor die 138″ and the top surface of the spring washers 182. Assuch, the emissions from the processor die 138″ are grounded to theground plane in the circuit board 132″ and suppressed directly at thesource of the emissions.

In the claims:
 1. An electronic assembly apparatus, comprising: acircuit board having a top surface and a bottom surface, the circuitboard including an electronic device emanating electromagneticemissions; an EMI reduction device that is coupled to the circuit board,the EMI reduction device is positioned around the outer periphery of theelectronic device, the EMI reduction device touches the circuit board atcontact points that surround the electronic device, the EMI reductiondevice has no physical contact with the electronic device; and a thermaldissipation device that is coupled to the EMI reduction device and thecircuit board, the thermal dissipation device is positioned over the topsurface of the circuit board; wherein the thermal dissipation devicedissipates heat generated from the circuit board; wherein the EMIreduction device reduces electromagnetic emissions generated from theelectronic device.
 2. The apparatus of claim 1 , the EMI reductiondevice including at least one mounting clip for attaching the EMIreduction device to the thermal dissipation device.
 3. The apparatus ofclaim 1 , the EMI reduction device including a plurality of springfingers that surround the electronic device and maintain physicalcontact with the circuit board.
 4. The apparatus of claim 1 , the EMIreduction device including an aperture that is positioned over theelectronic device; wherein the aperture enables the electronic device tomaintain thermal contact with the thermal dissipation device.
 5. Theapparatus of claim 1 , the thermal dissipation device having a topsurface and a bottom surface, the bottom surface including a thermalcompound; wherein the thermal compound is coupled to the electronicdevice.
 6. The apparatus of claim 1 , the EMI reduction device having atop surface and a bottom surface, the bottom surface having a dielectriclayer.
 7. An EMI reduction device for use with an electronic devicemounted on a circuit board, comprising: a frame that is coupled to thecircuit board, the frame surrounds peripheral portions of the electronicdevice, the frame is not in physical contact with the electronic device,the frame having a top surface and a bottom surface; and a plurality ofspring fingers that are coupled to the circuit board, the spring fingersare positioned onto the circuit board at points that surround theelectronic device.
 8. The apparatus of claim 7 , comprising: an aperturethat surrounds the electronic device; wherein the aperture enables theelectronic device to maintain thermal contact with a thermal dissipationdevice.
 9. The apparatus of claim 7 , comprising: a fastener that iscoupled to a thermal dissipation device; wherein the fastener enablesthe EMI reduction device to be detachably mounted to a thermaldissipation device.
 10. The apparatus of claim 7 , wherein theelectronic device is a processor.
 11. The apparatus of claim 7 , whereinthe bottom surface of the frame is non-electrically conductive.
 12. Theapparatus of claim 7 , wherein the top surface of the frame iselectrically conductive.
 13. A method for reducing EMI emissionsemanating from an electronic device having a ground plane, said methodcomprising the steps of: surrounding the outer periphery of theelectronic device with a frame that has no direct physical contact withthe electronic device; contacting the frame to the ground plane atcontact points that surround the electronic device; and capacitivelycoupling the EMI emissions from the electronic device to the groundplane.
 14. The method of claim 13 , comprising the steps of: placing athermal dissipation device over the electronic device; and dissipatingheat generated from the electronic device through the thermaldissipation device.
 15. The method of claim 14 , comprising the step of:attaching the frame to the thermal dissipation device.